Cover [Abstract] Entity Address, Address Line One Entity Address, Address Line Two Entity Address, Address Line Three Amendment Flag City Area Code Entity Address, City or Town Entity Address, Country Current Fiscal Year End Date Document Period End Date Entity [Domain] Entity Incorporation, State or Country Code Local Phone Number Entity Address, Postal Zip Code Entity Address, State or Province Title of 12(b) Security No Trading Symbol Flag Trading Symbol Security Exchange Name Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Current Reporting Status Entity Filer Category Entity Public Float Entity Registrant Name Entity Central Index Key Entity Tax Identification Number Entity Common Stock, Shares Outstanding Document Fiscal Year Focus Document Fiscal Period Focus Legal Entity [Axis] Document Type Document Annual Report Document Transition Report Entity Interactive Data Current Entity File Number Entity Shell Company Entity Emerging Growth Company Entity Small Business Award Type [Axis] Impairment losses related to real estate partnerships ASU 2016-07 [Member] ASU 2016-13 [Member] SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES [Abstract] ASU 2016-02 [Member] Receivable Type [Axis] Accumulated depreciation Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Net unrealized (loss) for pension and other postretirement obligations AOCI at year end Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax Accumulated other comprehensive (loss) Accumulated other comprehensive (loss) Accumulated Other Comprehensive Income (Loss), Net of Tax Accumulated Other Comprehensive Income/(Loss) By Component: Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract] Net unrealized gain (loss) on available-for-sale securities AOCI, Debt Securities, Available-for-sale, Adjustment, after Tax Accumulated Other Comprehensive Income/(Loss) [Member] AOCI Attributable to Parent [Member] Additional paid-in capital Additional Paid-in Capital [Member] Common stock issued under employee stock plan, tax benefits Adjustment to Additional Paid in Capital, Income Tax Effect from Share-based Compensation, Net Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to reconcile net income to net cash provided by operating activities [Abstract] Adjustments for New Accounting Pronouncements [Axis] Stock-based compensation APIC, Share-based Payment Arrangement, Increase for Cost Recognition Advertising costs Advertising All Other [Member] Stock-based compensation expense related to incentive and non-qualified stock options recognized Share-based Payment Arrangement, Expense Allowance for loan losses [Roll Forward] Charge-offs Allowance for Loan and Lease Losses, Write-offs Decrease in allowance for loan losses Allowance for Loan and Lease Losses, Period Increase (Decrease) Recoveries Allowance for Loan Losses by Class Financing Receivable, Allowance for Credit Loss [Table Text Block] Amortization of mortgage servicing rights Amortization of intangible assets Amortization of intangible assets Amortization of intangible assets Income attributable to unvested stock-based compensation awards Dilutive Securities, Effect on Basic Earnings Per Share, Options and Restrictive Stock Units Weighted-average anti-dilutive stock options outstanding (in shares) Total Assets, Fair Value Disclosure Assets: Assets: Assets [Abstract] Total assets Assets Total assets Supplemental Balance Sheet Information Related to Leases [Abstract] Gross unrealized gains Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Number of Positions [Abstract] Gross unrealized gains Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract] Amortized cost Debt Securities, Available-for-sale, Amortized Cost Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract] Available-for-Sale Portfolio [Abstract] Available-for-sale investment securities [Abstract] Available-for-sale investment securities, cost Amortized cost Gross unrealized losses Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax Gross unrealized losses Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Available-for-Sale, Debt Maturities, Amortized Cost [Abstract] Available-for-sale investment securities (cost of $3,011,551 and $2,952,278, respectively) Fair value Subtotal Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost Available-for-Sale, Debt Maturities, Fair Value [Abstract] Subtotal Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value Due in one year or less Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value Due after ten years Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] Due in one year or less Due after ten years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value Due after five years through ten years Without single maturity date Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost Due after one through five years Due after five years through ten years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value Due after one through five years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value Without single maturity date Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value Fair value Fair value Available-for-sale investment securities Balance Sheet Location [Domain] Balance Sheet Location [Axis] Income from bank-owned life insurance policies Bank Owned Life Insurance Income Time Deposits [Member] Banking [Member] Banking [Member] Building and Building Improvements [Member] Bank Premises [Member] Building [Member] Deferred tax liability Investment securities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities Community Bank System, Inc. common stock Total net consideration paid Purchase price of acquisition Other liabilities Other liabilities Other assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets Assets acquired Cash and cash equivalents Accrued interest and fees receivable Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables Issuance of common shares (in shares) Ownership interest Business Acquisition [Line Items] Total revenue, net of interest expense, pro forma Business Acquisition, Pro Forma Revenue Net income, pro forma Business Acquisition, Pro Forma Net Income (Loss) Business Acquisition [Axis] ACQUISITIONS [Abstract] Business Acquisition, Acquiree [Domain] Supplemental Pro Forma Financial Information [Abstract] Pro Forma Financial Information Premises and equipment Consideration paid [Abstract] Business Combination, Consideration Transferred [Abstract] Business development and marketing ACQUISITIONS Business Combination Disclosure [Text Block] Intangibles Total identifiable assets, net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Acquisition expenses Acquisition expenses Merger and acquisition integration related (recoveries) expenses Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract] Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest [Abstract] Estimated fair value of assets and liabilities assumed in acquisition [Abstract] Acquisitions [Abstract] Total risk-based capital, actual amount Total risk-based capital [Abstract] Total risk-based capital, actual ratio Total risk based capital to be well-capitalized under prompt corrective action, amount Total risk based capital to be well-capitalized under prompt corrective action, ratio Total risk-based capita for capital adequacy purposes, ratio Total risk-based capital for capital adequacy purposes, amount Carrying Value [Member] Cash and Cash Equivalents [Abstract] Cash and Cash Equivalents [Axis] Supplemental Cash Flow Information Related to Leases [Abstract] Cash Flow, Operating Activities, Lessee [Abstract] Change in cash and cash equivalents Change in cash and cash equivalents Cash acquired from acquisition Cash acquired related to acquisition Cash and cash equivalents Cash and cash equivalents at end of year Cash and cash equivalents at beginning of year Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Cash and Cash Equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Supplemental disclosures of noncash financing and investing activities: Supplemental disclosures of noncash financing activities [Abstract] Summary of loans acquired [Abstract] Certain Loans Acquired in Transfer Not Accounted for as Debt Securities [Abstract] Net reclassification from non-accretable to accretable Kinderhook acquisition Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield, Additions Balance at end of year Balance at beginning of year Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Accretion recognized Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield, Accretion Accretable discount related to acquired loans [Roll forward] Fair value of acquired loans Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value Expected cash flows at acquisition Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Cash Flows Expected to be Collected at Acquisition Contractually required principal and interest at acquisition Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Contractually Required Payments Receivable at Acquisition Business Lending [Member] Commercial Loan [Member] Commercial Portfolio Segment [Member] Commitments and contingencies (See Note N) Commitments and Contingencies Commitments to Extend Credit [Member] COMMITMENTS, CONTINGENT LIABILITIES AND RESTRICTIONS [Abstract] COMMITMENTS, CONTINGENT LIABILITIES AND RESTRICTIONS Commitments and Contingencies Disclosure [Text Block] Common Stock Amount Issued [Member] Common Stock [Member] Common equity Tier 1 capital, actual amount Common Equity Tier One Capital Cash dividends declared per share (in dollars per share) Dividends declared per common share (in dollars per share) Common equity Tier 1 capital, actual ratio Common Equity Tier One Capital Ratio Common equity Tier 1 capital for capital adequacy purposes, amount Common Equity Tier One Capital Required for Capital Adequacy Common equity Tier 1 capital to be well-capitalized under prompt corrective action, amount Common Equity Tier One Capital Required to be Well-Capitalized Common equity Tier 1 capital [Abstract] Common stock, par value (in dollars per share) Common stock, shares issued (in shares) Common stock, shares authorized (in shares) Common stock, $1.00 par value, 75,000,000 shares authorized; 51,974,726 and 51,576,839 shares issued, respectively Balance (in shares) Balance (in shares) Common Stock, Shares, Outstanding Data processing and communications Compensation Related Costs [Abstract] Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] Components of net deferred tax liability [Abstract] Comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Hardware [Member] Computer Equipment [Member] PARENT COMPANY STATEMENTS Condensed Financial Information of Parent Company Only Disclosure [Text Block] Principles of Consolidation Consumer Portfolio Segment [Member] Contract Balances [Abstract] Accounts receivable Core Deposits [Member] Core Deposit Intangibles [Member] Corporate Debt Securities [Member] Credit Loss Status [Axis] Credit Loss Status [Domain] Cumulative effect of change in accounting principle - equity securities Increase in retained earnings State and other Current State and Local Tax Expense (Benefit) Federal Current Federal Tax Expense (Benefit) Current [Abstract] Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Customer Relationships [Member] Customer Lists [Member] Doubtful [Member] Designated as Hedging Instrument [Member] Gross realized losses 12 months or longer Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer Total Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions Proceeds from sales of securities [Abstract] Total Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss Total Debt Securities, Available-for-sale, Unrealized Loss Position 12 months or longer Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Number of Positions Less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months Less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Number of Positions Gross realized gains Weighted-average rate Issuance date 12 months or longer Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Less than 12 months Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss Fair Value of Subordinated Notes Payable [Abstract] Debt Instrument, Fair Value Disclosure [Abstract] Variable interest rate, basis spread BORROWINGS [Abstract] Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] BORROWINGS Debt Disclosure [Text Block] Call price Debt Instrument, Call Feature Variable interest rate basis Debt Instrument [Line Items] Debt Instrument, Name [Domain] Par amount Debt Instrument, Face Amount Fixed interest rate percentage Maturity date Effective interest rate Subordinated notes payable, premium Debt Instruments [Abstract] Pension Deferred Tax Liabilities, Prepaid Expenses Shares held by deferred compensation arrangements (in shares) Deferred Compensation Arrangement with Individual, Common Stock Reserved for Future Issuance Expense recognized under plan Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items] Other Postretirement Benefits, Individual Contracts, Type of Deferred Compensation [Axis] Deferred compensation arrangements (179,548 shares at December 31, 2019 and 207,403 shares at December 31, 2018) Liability recorded for compensation plan Liability accrued for compensation plan Expense recognized under plan Deferred Compensation Arrangement with Individual, Compensation Expense Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items] Deferred compensation arrangement with individual, shares credited (in shares) Deferred Compensation Arrangements [Member] Equity-Based Arrangements, Individual Contracts, Type of Deferred Compensation [Axis] Deferred Compensation Arrangements [Abstract] Federal Deferred [Abstract] Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Deferred tax liability Deferred Tax Liabilities, Gross State and other Deferred tax asset Deferred Tax Assets, Net of Valuation Allowance Employee benefits Other, net Net deferred tax liability Deferred Tax Liabilities, Net Allowance for loan losses Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Provision for Loan Losses Depreciation Deferred Tax Liabilities, Property, Plant and Equipment Goodwill and intangibles Deferred Tax Liabilities, Goodwill Mortgage servicing rights Deferred Tax Liabilities, Mortgage Servicing Rights Pension Plans [Abstract] Pension Plan [Abstract] Participant contributions Benefits paid Defined Benefit Plan, Benefit Obligation, Benefits Paid U.S. Large-Cap [Member] Defined Benefit Plan, Equity Securities, US, Large Cap [Member] Equity Securities [Member] Defined Benefit Plan, Equity Securities [Member] Money Market Accounts / Cash Equivalents [Member] Benefits paid Defined Benefit Plan, Plan Assets, Benefits Paid Participant contributions Defined Benefit Plan, Plan Assets, Contributions by Plan Participant Net loss Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year Total Defined Benefit Plan, Expected Amortization, Next Fiscal Year International [Member] Target allocation percentage of assets Defined Benefit Plan, Plan Assets, Target Allocation, Percentage Prior service credit Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year CBU Stock [Member] Defined Benefit Plan, Funding Status [Extensible List] 2020 Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months 2024 Defined Benefit Plan, Expected Future Benefit Payment, Year Five 2022 Defined Benefit Plan, Expected Future Benefit Payment, Year Three 2023 Defined Benefit Plan, Expected Future Benefit Payment, Year Four Deferred actuarial (gain)/loss Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 2025-2029 2021 Defined Benefit Plan, Expected Future Benefit Payment, Year Two Defined Contribution Plan [Table] Defined Contribution Plan Disclosure [Line Items] Amortization of unrecognized net loss/(gain) Defined Benefit Plan, Amortization of Gain (Loss) BENEFIT PLANS [Abstract] Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract] Actual return of plan assets Net loss Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax Costs that will be amortized from accumulated other comprehensive (income) loss in net periodic (income) cost in next fiscal year [Abstract] Pre-tax AOCI Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax Amounts recognized in the consolidated statement of condition [Abstract] Defined Benefit Plan Disclosure [Line Items] Amortization of prior service cost Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Change in benefit obligation [Roll Forward] Rate of compensation increase Rate of compensation increase Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase Benefit obligation at the beginning of year Benefit obligation at end of year Defined Benefit Plan, Benefit Obligation Contribution made to defined benefit pension plan by employer Employer contributions Defined Benefit Plan, Plan Assets, Contributions by Employer Plan amendment / acquisition Defined Benefit Plan, Benefit Obligation, Business Combination Change in plan assets [Roll Forward] Plan acquisition Expected return on plan assets Discount rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate Discount rate Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate Net periodic benefit cost [Abstract] Expected return on plan assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Estimated Future Benefit Payments [Abstract] Fair value of plan assets Fair value of plan assets at beginning of year Fair value of plan assets at end of year Defined Benefit Plan, Plan Assets, Amount Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] Net periodic (benefit) Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Plan Assets [Abstract] Interest cost Over/(Under) funded status at year end Defined Benefit Plan, Funded (Unfunded) Status of Plan (Income)/expense recognized under plan Weighed average assumption used in calculating benefit obligations [Abstract] Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] Plan amendment Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment Service cost Defined Contribution Plan [Abstract] Other assets Assets for Plan Benefits, Defined Benefit Plan Defined Benefit Plan, Plan Assets, Category [Axis] Net prior service cost (credit) Deposits Deposit Liabilities, Type [Table Text Block] DEPOSITS Deposit Liabilities Disclosures [Text Block] Total deposits Deposits Savings Money market Deposits Deposits, Fair Value Disclosure DEPOSITS [Abstract] Depreciation Weighted average maturity period Derivative [Line Items] Derivative Instruments [Abstract] Derivative Instrument Detail [Abstract] Derivative asset Derivative [Table] DERIVATIVE INSTRUMENTS [Abstract] Derivative Instrument [Axis] DERIVATIVE INSTRUMENTS Derivative Instruments and Hedging Activities Disclosure [Text Block] Derivative liability Derivative Liability Fair Values of Derivative Instruments [Abstract] Derivative weighted average pay rate Derivative, Fixed Interest Rate Derivative assets Derivative Asset, Fair Value, Gross Asset Derivative liabilities Hedging Relationship [Axis] Derivative, notional amount Derivative weighted average receive rate Derivative, Variable Interest Rate Derivative Contract [Domain] Derivative Financial Instruments and Hedging Activities Derivatives, Policy [Policy Text Block] STOCK-BASED COMPENSATION PLANS [Abstract] STOCK-BASED COMPENSATION PLANS Share-based Payment Arrangement [Text Block] LIMITS ON DIVIDENDS AND OTHER REVENUE SOURCES [Abstract] Common Dividends, Common Stock Dividends declared: Cash dividends declared: Dividends [Abstract] Dividends declared and unpaid Cash due from banks Reconciliation of basic to diluted earnings per share [Abstract] Earnings Per Share Reconciliation [Abstract] EARNINGS PER SHARE Earnings Per Share [Text Block] Diluted earnings per share (in dollars per share) Earnings Per Share, Diluted Basic earnings per share (in dollars per share) Earnings Per Share, Basic Earnings Per Share Basic earnings per share [Abstract] Earnings Per Share, Basic [Abstract] Diluted earnings per share [Abstract] Earnings Per Share, Diluted [Abstract] EARNINGS PER SHARE [Abstract] Federal corporate tax rate Federal statutory income tax rate State income taxes, net of federal benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent Stock-based compensation Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Payment Arrangement, Percent Other, net Effective Income Tax Rate Reconciliation, Other Adjustments, Percent Federal tax credits Effective Income Tax Rate Reconciliation, Tax Credit, Percent Effective income tax rate Effective Income Tax Rate Reconciliation, Percent Tax-exempt interest Effective Income Tax Rate Reconciliation, Tax Exempt Income, Percent Effective tax rate reconciliation [Abstract] Income tax benefit recognized Related tax benefits from stock option exercises Share-based Payment Arrangement, Exercise of Option, Tax Benefit Unrecognized stock-based compensation expense Share-based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount Unrecognized stock-based compensation expense related to non-vested stock options Weighted-average period for recognition of unrecognized compensation cost Weighted average period for recognition of unrecognized compensation cost Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Employee Stock Ownership Plan (ESOP) Name [Axis] Remaining Stock Options [Member] Stock Options [Member] Unrecognized Stock-Based Compensation Expense and Stock Option Exercises [Abstract] Share-based Payment Arrangement, Additional Disclosure [Abstract] Employee Stock Ownership Plan (ESOP), Plan [Domain] Equipment [Member] Equity-Based Arrangements, Individual Contracts, Type of Deferred Compensation [Domain] Amortized cost Equity Securities, FV-NI, Cost Fair value Equity securities Equity Securities, FV-NI Equity Component [Domain] Unrealized gain on equity securities Unrealized gain on equity securities Equity Securities, at Fair Value [Member] Equity Securities [Member] Fair Value [Member] Withholding taxes paid on share-based compensation Extension of Term [Member] Extended Maturity [Member] Assets and Liabilities Measured on Nonrecurring Basis [Abstract] Fair Value Measurements, Nonrecurring Value Measurement [Abstract] Total Fair Value, Net Asset (Liability) Significant Unobservable Inputs used in determination of Fair Value of Assets Classified as Level 3 [Abstract] FAIR VALUE [Abstract] Contract amount of commitments and contingencies Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Recurring [Member] Fair Value, Measurement Frequency [Domain] Non-recurring [Member] Fair Value, Nonrecurring [Member] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Axis] Asset Class [Domain] Asset Class [Axis] Fair Value Measurement Inputs and Valuation Techniques [Table] Measurement Frequency [Axis] Measurement Basis [Axis] FAIR VALUE Fair Value Disclosures [Text Block] Fair Values of Financial Instruments Fair value of assets acquired, excluding acquired cash and intangibles Assets and Liabilities Measured on a Non-Recurring Basis Fair Value Measurements, Nonrecurring [Table Text Block] Significant Unobservable Inputs, Fair Value Valuation Techniques Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Valuation Techniques [Table Text Block] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, by Balance Sheet Grouping [Table] Carrying Amounts and Estimated Fair Values of Other Financial Instruments Fair Value, by Balance Sheet Grouping [Table Text Block] Fair Value Measurement [Domain] Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] Level 1 [Member] Significant Unobservable Inputs, Level 3 [Member] Level 3 [Member] Significant Observable Inputs, Level 2 [Member] Level 2 [Member] Subordinated debt held by unconsolidated subsidiary trusts Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items] Fair Value Hedging [Member] FDIC insurance premiums Other Federal Home Loan Bank borrowings Other FHLB borrowings Federal Home Loan Bank unused borrowing capacity Loans Financing Receivable [Policy Text Block] Financial Instrument Performance Status [Domain] Financial Instrument Performance Status [Axis] Acquired Impaired [Member] Financial Asset Acquired with Credit Deterioration [Member] Financial Instrument [Axis] Financial Instrument [Axis] Financial liabilities [Abstract] Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract] Financial assets [Abstract] Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract] Total past due Loans modified in TDR during the year, number Financing Receivable, Recorded Investment [Line Items] Financing Receivable Portfolio Segment [Axis] Loans by Credit Quality Indicator Financing Receivable Credit Quality Indicators [Table Text Block] TDRs, amount Financing Receivable, Modifications [Line Items] Class of Financing Receivable, Type [Domain] Class of Financing Receivable, Type [Domain] Financing Receivables, Period Past Due [Domain] Loans modified in TDR during the year, amount Nonaccrual Class of Financing Receivable, Type [Axis] Class of Financing Receivable, Type [Axis] Current Financing Receivables, Period Past Due [Axis] Aged analysis of the company's loans [Abstract] 90 + days past due and still accruing Financing Receivable Portfolio Segment [Domain] Financing Receivable, Recorded Investment, Past Due [Line Items] Financing Receivable, Allowance for Credit Losses [Line Items] Period of amortization Gross carrying amount Net carrying amount Finite-Lived Intangible Assets, Net Finite-Lived Intangible Assets, Major Class Name [Domain] Thereafter Finite-Lived Intangible Assets, Amortization Expense, after Year Five Accumulated amortization Finite-Lived Intangible Assets, Accumulated Amortization Intangible Assets [Abstract] Identifiable Intangible Assets [Abstract] Finite-Lived Intangible Assets, Net [Abstract] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets by Major Class [Axis] 2020 Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months Estimated aggregate amortization expense for each of five succeeding fiscal years [Abstract] Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] Estimated useful life Intangible asset useful life (amortization period) 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2024 Finite-Lived Intangible Assets, Amortization Expense, Year Five 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Three 2023 Finite-Lived Intangible Assets, Amortization Expense, Year Four Fixed Income Securities [Member] Fixed Income Securities [Member] Forward Sales Commitments [Member] Forward Contracts [Member] Loss on debt prepayment Loss on debt extinguishment Loss on debt extinguishment Accumulated impairment, activity Goodwill, Impairment Loss Goodwill, activity Goodwill, Written off Related to Sale of Business Unit Goodwill, net, activity Goodwill, Period Increase (Decrease) Goodwill, activity Goodwill, end of period Goodwill, beginning of period Goodwill, Gross GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS Goodwill and Intangible Assets Disclosure [Text Block] Goodwill, net Goodwill Goodwill, net, end of period Goodwill, net, beginning of period Goodwill Increase (decrease) in goodwill as a result of fair value adjustment Components of goodwill [Abstract] Goodwill [Roll Forward] GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS [Abstract] Accumulated impairment, end of period Accumulated impairment, beginning of period Goodwill, Impaired, Accumulated Impairment Loss Intangible Assets Cumulative amount of fair value hedging adjustment included in the carrying amount of the hedged assets Carrying amount of the hedged assets Hedging Designation [Axis] Hedging Relationship [Domain] Hedging Designation [Domain] Eliminations [Member] Intersegment Eliminations [Member] Average impaired loans Non-Business Impaired Loans Impaired Financing Receivables [Table Text Block] Interest income recognized Loans with allowance allocation Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance Loans without allowance allocation Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance Specifically allocated allowance Valuation allowance Impaired Financing Receivable, Related Allowance Carrying balance Impaired Financing Receivable, Unpaid Principal Balance INCOME TAXES [Abstract] Equity in undistributed net income of subsidiaries Equity in undistributed net income of subsidiaries Income (Loss) from Equity Method Investments CONSOLIDATED STATEMENTS OF INCOME [Abstract] Condensed Statements of Income [Abstract] INCOME TAXES Income Tax Disclosure [Text Block] Income before income taxes Income before tax benefit and equity in undistributed net income of subsidiaries Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Income taxes Income tax benefit Provision for income taxes Income Tax Expense (Benefit) Provision for income taxes [Abstract] Income Tax Expense (Benefit), Continuing Operations [Abstract] Income Taxes Cash paid for income taxes Decrease in other assets Increased in accrued interest and fees receivable (Benefit)/provision for deferred income taxes Increase (Decrease) in Deferred Income Taxes Increase in deferred compensation arrangements Increase in deferred compensation arrangements Net increase (decrease) in deposits Increase in deposits Decrease in loans Increase (Decrease) in Finance Receivables Change in other assets and liabilities Net change in other assets and other liabilities Increase (Decrease) in Other Operating Assets and Liabilities, Net Increase in other liabilities Increase (Decrease) in Other Operating Liabilities Increase (Decrease) in Stockholders' Equity [Roll Forward] Assumed exercise of stock options (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Insurance services Insurance Services Revenue Core deposit intangibles & Other intangibles Intangible Assets, Net (Excluding Goodwill) Interest and fees on loans Interest and dividends on investments Interest income: Total interest income Interest and Dividend Income, Operating Interest on money market Interest on Deposits [Abstract] Interest Expense Domestic Deposit Liabilities [Abstract] Interest on savings Interest on time Interest on interest checking Interest expense: Total interest expense Interest Expense Interest on subordinated debt held by unconsolidated subsidiary trusts Interest on borrowings Interest Expense, Junior Subordinated Debentures Interest on deposits Total interest on deposits Interest on subordinated notes payable Net interest income Net interest income Interest and dividends on taxable investments Interest and dividends on nontaxable investments Net interest income after provision for loan losses Interest Income (Expense), after Provision for Loan Loss Cash paid for interest Accrued interest and fees receivable Interest Receivable Interest Rate Swap Agreements [Member] Interest Rate Swaps [Member] Interest-bearing deposits Interest checking Internal Credit Assessment [Axis] Internal Credit Assessment [Axis] Internal Credit Assessment [Domain] Investments [Domain] Amortized Cost and Estimated Fair Value of Investment Securities Investment [Table Text Block] Federal Home Loan Bank Common Stock [Member] Investment in Federal Home Loan Bank Stock [Member] Investment Type [Axis] INVESTMENT SECURITIES Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] INVESTMENT SECURITIES [Abstract] Investments in Real Estate Limited Partnerships [Abstract] Investment securities Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity Investment in and advances to [Abstract] Subordinated debt held by unconsolidated subsidiary trusts Borrowings Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust Terms of preferred securities for each trust [Abstract] Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust [Abstract] Subordinated notes Subordinated notes payable, net of premium of $795 and $0, respectively Subordinated notes payable LIBOR [Member] Operating lease that has not yet commenced [Abstract] Lessee, Lease, Description [Line Items] Thereafter Lessee, Operating Lease, Liability, Payments, Due after Year Five 2020 Lessee, Operating Lease, Liability, Payments, Due Next Twelve Months 2023 Lessee, Operating Lease, Liability, Payments, Due Year Four 2024 Lessee, Operating Lease, Liability, Payments, Due Year Five Lease term Total lease payments Lessee, Operating Lease, Liability, Payments, Due Term of option to extend Lessee, Operating Lease, Renewal Term Maturities of Lease Liabilities Lessee, Operating Lease, Liability, Maturity [Table Text Block] Operating Leases [Abstract] Lessee, Operating Lease, Description [Abstract] Lessee, Lease, Description [Table] Less imputed interest Lessee, Operating Lease, Liability, Undiscounted Excess Amount Leases Lessee, Leases [Policy Text Block] 2022 Lessee, Operating Lease, Liability, Payments, Due Year Three 2021 Lessee, Operating Lease, Liability, Payments, Due Year Two Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Salaries and employee benefits Land and Land Improvements [Member] Land Improvements [Member] Components of Lease Expense [Abstract] Lease, Cost [Abstract] Total lease cost Lease, Cost Components of Lease Expense LEASES Leases of Lessee Disclosure [Text Block] LEASES [Abstract] Fair value of liabilities assumed Liabilities: Liabilities [Abstract] Increase (decrease) in other liabilities as a result of reclassification and adjustment Liabilities and shareholders' equity [Abstract] Liabilities and Equity [Abstract] 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[Abstract] Operating activities: Operating activities [Abstract] Net cash used in financing activities Net Cash Provided by (Used in) Financing Activities Net income Net income Income available to common shareholders Net Income (Loss) Available to Common Stockholders, Basic Financing activities: Financing activities [Abstract] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Adjustments for New Accounting Pronouncement [Member] New Accounting Pronouncements [Abstract] Recently Adopted Accounting Pronouncements [Abstract] New Accounting Pronouncements or Change in Accounting Principle [Table] Acquisitions: Noninterest-bearing deposits Total noninterest revenues Noninterest revenue Noninterest Income Noninterest checking Noninterest expenses: Expenses [Abstract] Total noninterest expenses Noninterest Expense Noninterest revenues: Loans receivable, net [Abstract] Not Designated as Hedging Instrument [Member] Occupancy and equipment Maturities of Lease Liabilities [Abstract] Operating leases Weighted average remaining lease term operating lease Operating Lease, Weighted Average Remaining Lease Term Operating lease right-of-use assets Operating lease cost Operating Lease, Liability, Statement of Financial Position [Extensible List] Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List] Rental expense Direct expenses Operating Segments [Member] Operating leases Weighted average discount rate operating lease Operating Lease, Weighted Average Discount Rate, Percent Operating cash outflows for operating leases Operating lease liabilities Total 2020 Future minimum rental commitments [Abstract] 2023 2022 2021 Total Operating Leases, Future Minimum Payments Due 2019 Thereafter Other real estate owned, Valuation Technique [Extensible List] Other real estate owned, measurement input Other real estate owned, Measurement Input [Extensible List] Reclassification adjustment for net (gains) included in net income, gross Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax Other Postretirement Benefits, Individual Contracts, Type of Deferred Compensation [Domain] Amortization of actuarial losses included in net periodic pension cost, net Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax Tax effect Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, Tax Amortization of prior service cost included in net periodic pension cost, net Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, after Tax Amortization of actuarial losses included in net periodic pension cost, gross Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax Tax effect Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax Reclassification adjustment for net (gains) included in net income, net Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax Tax effect Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, Tax Amortization of prior service cost included in net periodic pension cost, gross Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax Other Real Estate [Abstract] Other Intangibles [Member] Other Intangibles [Member] Net (gain) loss Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, after Tax Prior service cost/(credit) Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Tax Other assets Other assets Other Assets Amounts recognized in AOCI, net of tax [Abstract] Net unrealized holding gains (losses) arising during period, net Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax Other operating expenses Tax effect Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax Net unrealized holding gains (losses) arising during period, gross Amortized cost Other Investments and Securities, at Cost Other expenses Other expenses Other real estate owned Post-retirement Benefits [Member] Other Postretirement Benefits Plan [Member] Other real estate Other Assets [Member] Other Investments [Member] Other Investments [Member] Other comprehensive income/(loss), net of tax Other comprehensive income/(loss) Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Other comprehensive income/(loss), net of tax [Abstract] Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Unrealized gains/(losses) on available-for-sale securities: Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax, Portion Attributable to Parent [Abstract] Other comprehensive (loss)/income related to pension and other post retirement obligations, net of taxes Changes in other comprehensive (loss)/income related to pension and other post retirement obligations Total Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax and Reclassification Adjustment, Attributable to Parent Pension and other post retirement obligations: Other comprehensive gain (loss) related to unrealized gains/(losses) on available-for-sale securities, net of taxes Changes in other comprehensive income/(loss) related to unrealized losses on available-for-sale securities Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax, Portion Attributable to Parent Performing [Member] Performing Financial Instruments [Member] Aged Analysis of Past Due Loans by Class Financing Receivable, Past Due [Table Text Block] Pass [Member] Waiver of Payments for Period of Time [Member] Payment Deferral [Member] Return of capital from/(capital contributions to) Payments for (Proceeds from) Businesses and Interest in Affiliates Net (increase) decrease in loans Payments for (Proceeds from) Loans and Leases Real estate limited partnership investments Payments for (Proceeds from) Real Estate Partnership Investment, Net Purchase of treasury stock Payments for Repurchase of Common Stock Cash dividends paid Payments of Ordinary Dividends, Common Stock Cash paid for acquisition, net of cash acquired of $90,381, $16, and $52,132, respectively Cash Cash paid for acquisitions, net of cash acquired of $1,328, $0, and $150,534, respectively Payments to Acquire Businesses, Net of Cash Acquired Cash dividends paid Payments of Ordinary Dividends Purchases of available-for-sale investment securities Payments to Acquire Debt Securities, Available-for-sale Purchases of premises and equipment, net Payments to Acquire Property, Plant, and Equipment Pension Benefits [Member] Pension Benefits [Member] Retirement Benefits BENEFIT PLANS